Home / Questions / 262466
Accepted
Aritzbn
Asked: 2020-05-11 01:58:26 +0800 CST 2020-05-11 01:58:26 +0800 CST

Update Layout from external thread

  • 772

I am trying to update a LinearLayout from a class outside the activity and doing so gives me the following:exception

Android.Util.AndroidRuntimeException:"Only the original thread that created a view hierarchy can touch its views."

This is what I have:

The first thing I do is launch a thread that every few seconds will check that I have a connection to the server.

protected override void OnCreate(Bundle savedInstanceState)
{
    base.OnCreate(savedInstanceState);
    SetContentView(Resource.Layout.activity_main);
    this.RequestedOrientation = Android.Content.PM.ScreenOrientation.Portrait;
    context = this;

    Task.Run( () => ComprobarConexion());
}

This is the method I use:

protected static void ComprobarEstado()
{
    while(true){
        TestearConexion();
        Thread.Sleep(segundos * 1000);
    }
}

And here I check that the connection is working:

public static void TestearConexion()
{
     try{ 

         WebService.Test();

     }catch(Exception ex){ //En la realidad tengo completadas distintas excepciones.

          LayoutInflater lDesconexion = (LayoutInflater)MainActivity.context.GetSystemService(Context.LayoutInflaterService);
          View vDesconexion = lDesconexion.Inflate(Resource.Layout.layout_disconected, null, false);
          MainActivity.contenidoPrincipal.RemoveAllViews();
          MainActivity.contenidoPrincipal.AddView(vDesconexion);

     }
}

The exception I get when trying to modify MainActivity.contenidoPrincipal.

How can I show the connection failure?

c#

1 Answers

  1. Best Answer

    First, you must understand how a Task and a Thread work to simulate asynchronous execution.

    Not recommended: Thread.Sleep

    Thread.Sleep: is the classic way to suspend the execution. This method will suspend the current thread until the specified time has elapsed. When you call Thread.Sleep, there is nothing you can do to abort this except wait until the timer elapses or by restarting the application. That's because it Thread.Sleep suspends the thread that is making the call .

    Recommended: Task.Delay

    It acts in a very different way than Thread.Sleep. Basically, Task.Delay will create a task that will complete after a time delay . Task.Delayit doesn't block the calling thread, so the UI will still be responsive .

    Difference in memory between Thread.Sleep and Task.Delay:

    The problem Thread.Sleepis, that during the time the Thread is idle, it cannot be used for another task . Losing thread dose doesn't necessarily mean we're wasting CPU time, but there is a cost to creating new threads and keeping an unused thread alive. This is not important in desktop applications, because there is a lot of memory available, but in servers or mobile applications, yes, since you have to conserve RAM memory and that is very important. So instead of Thread.Sleepit is recommended to use Task.Delay. By doing that when we expect, the thread can be used for other tasks. Which means that while we wait, the thread is released back to its caller or thread pool andno resources are wasted in the process .

    Understood the difference, here we go with the solution:

    Solution:

    protected override void OnCreate(Bundle savedInstanceState)
{
    base.OnCreate(savedInstanceState);
    SetContentView(Resource.Layout.activity_main);
    this.RequestedOrientation = Android.Content.PM.ScreenOrientation.Portrait;

    // Crea una variable estatica tipo Activity
    Activity = this;

    ComprobarConexionAsync().GetAwaiter();
}


protected static async Task ComprobarConexionAsync()
{
    int factor = 1; // Factor a multiplicar del tiempo
    int intentos = 0; // Intentos actuales
    int limite = 5; // limites intentos de conexión como máximo

    void MostrarConexionFallida() 
    {
        // No es necesario, porque estas en un Task, pero para salvarte correr en el UI
          MainActivity.Activity?.RunOnUiThread(() => 
          {
                Toast.MakeText(MainActivity.Activity, "No se puede conectar con el servidor", ToastLength.Long).Show();
          });
    }

    // Mientras no se compruebe la conexion
    while(!TestearConexion())
    {
        await Task.Delay(++factor * 1000);
        if(++intentos >= limite){
           MostrarConexionFallida();
           return;
        }
    }

   // En este punto hay conexión
}

public static bool TestearConexion()
{
     bool conectado = true;
     try
     { 
       WebService.Test();

     } 
     catch(Exception ex)
     { 
          conectado = false;
     }

   return conectado;
}

    Recommendation: if you are going to use a method to check a WebService connection you should create a utility Singleton class and not attach it to an Activity. Activities tend to take on a lot of tasks and your code can get polluted with useful code.

    References and examples:

    • 3